The present invention relates to an objective lens for an optical information recording/reproducing apparatus adapted to record information to and/or reproduce information from two types of optical discs based on different standards, and to an optical information recording/reproducing apparatus on which such an objected lens is mounted.
There exist various standards of optical discs, such as DVD (Digital Versatile Disc) and BD (Blu-ray Disc), differing in recording density, protective layer thickness, etc. Therefore, an objective optical system mounted on the optical information recording/reproducing apparatus is required to have a compatibility with a plurality of types of optical discs. In this case, the term “compatibility” means to guarantee realizing information recording and information reproducing without the need for replacement of components even when the optical disc being used is changed. Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording.
In order to have the compatibility with the plurality of types of optical discs based on the different standards, it is necessary to correct the spherical aberration which changes depending on the difference in protective layer thickness (i.e., a distance between a recording surface and a surface of a protective layer on an optical disc) between the optical discs and to form a suitable beam spot in accordance with the difference in recording density between the optical discs by changing the numerical aperture NA of the objective optical system. In general, the size of the beam spot becomes smaller, as the wavelength of a laser becomes shorter. For this reason, the optical information recording/reproducing apparatus is configured to selectively use one of a plurality of types of laser beams having different wavelengths in accordance with the recording density of the optical disc being used. The optical information recording/reproducing apparatus uses, for example, light having the wavelength of approximately 660 nm (i.e., so-called red laser light) for information recording or information reproducing for DVD and light having the wavelength of approximately 405 nm (i.e., so-called blue laser light) for information recording or information reproducing for BD.
Each of Japanese Patent Provisional Publications No. 2004-265573A and No. 2004-326862A discloses an optical information recording/reproducing apparatus having the compatibility with BD and DVD. The optical information recording/reproducing apparatus is provided with an objective lens configured to have a diffraction structure having a plurality of annular zones concentrically formed about an optical axis. The diffraction structure has a first region which contributes to convergence of blue laser light on a recording surface of BD and convergence of red laser light on a recording surface of DVD, and has a second region which contributes to only convergence of the blue laser light on the recording surface of BD. In order to increase the diffraction efficiency for each of the laser beams based on the different standards, the optical information recording/reproducing apparatus is configured to use different high-orders of diffracted light respectively for the laser beams based on the different standards. However, the diffraction structure disclosed in the above described publications has a drawback that since the height of each step is high, processing and molding for a lens is difficult.
To solve the above described drawback, it is necessary to set the diffraction order (at which the diffraction efficiency is maximized) to a low order for each of the laser beams based on the different standards. However, when the diffraction order is set to a low order, the diffraction efficiency inevitably decreases when one of the laser beams based on the different standards is used. For this reason, the inventors of the present invention focused attention to the fact that the diffraction structure should be designed to use the same low diffraction orders of light for the laser beams based on the different standards.
However, if the first region is designed to use the same low diffraction orders of light for the laser beams based on the different standards, the diffraction efficiency of the blue laser beam in the first region becomes smaller than that in the second region. Since the transmission light amount of the blue laser beam shows nonuniformity between the pupil center and the periphery of the pupil, the blue laser beam is affected by super-resolution. The term “super-resolution” means a phenomenon where the beam spot size is reduced when the transmission light amount in the peripheral of the pupil is larger than the transmission light amount in the pupil center. Therefore, when super-resolution is caused, the beam spot size formed on the recording surface of the optical disc is considerably reduced relative to an expected beam spot size. In this case, since the beam spot size is too small with respect to a pit on the recording surface, the information recording or information reproducing may not be executed properly.
An optical information recording/reproducing apparatus disclosed in Japanese Patent Provisional Publication No. HEI 7-98431A (hereafter, referred to as JP HEI 7-98431A) is configured to solve a problem that the transmission light amount has nonuniformity between the pupil center and the periphery of the pupil, by utilizing a property that the light amount of a laser beam becomes lower at a point closer to the periphery of the laser light. Specifically, in JP HEI 7-98431A, a designer of the optical information recording/reproducing apparatus tries to reduce the amount of light entering the peripheral region of the objective optical system, by setting the acceptance angle of a collimator lens to a wide value and thereby setting the transmission light amount in the central region to be close to the transmission light amount in the peripheral region. However, in this case, the design restrictions regarding the focal length of the collimator lens becomes severe, and thereby it becomes difficult to freely set the magnification of the objective optical system.